The present invention relates to electrochemical processes for production of chlorine and caustic from brine and to the simultaneous production of electrical energy. More particularly, the invention is directed to the treatment of cell liquor from chloralkali cell to separate the sodium ions from the cell liquor and concentrate them in another liquor to form a sodium hydroxide solution.
The production of chlorine and crude caustic solutions by electrolysis of brine is a major industry. Two types of electrolysis cells are used in the production of chlorine and caustic; the diaphragm cell and the membrane cell. Considerable quantities of energy are required for electrolysis of the brine to produce chlorine and subsequent treatment of the cell liquor resulting from electrolysis to obtain caustic solutions of the desired purity and concentration. A 50 weight percent aqueous caustic solution of low sodium chloride content is a commercially desired product.
Known processes for electrolysis of brine produce cathode cell liquors having a caustic content of about 10 to 15 percent by weight, occasionally as high as about 25 percent by weight, and a sodium chloride content of up to about 15 percent by weight. The cell liquor produced by a diaphragm cell typically contains about 10 to 12 percent by weight caustic (NaOH) and 15 percent by weight sodium chloride (NaCl).
In the diaphragm cell, brine is continuously fed to an anode compartment, where chlorine is produced, and then flows through a diaphragm, usually made of asbestos, to a cathode compartment. Hydrogen gas is discharged from the solution at the cathode, with attendant generation of hydroxyl ions. To minimize back-migration of hydroxide ions from the cathode compartment to the anode compartment, a positive flow rate is always maintained; that is, a flow in excess of the conversion rate. As a consequence, the resulting catholyte solution, i.e., the cathode cell liquor as the term is used herein, has unconsumed sodium chloride in addition to product sodium hydroxide. The cathode cell liquor containing the sodium hydroxide and sodium chloride must be purified and concentrated to obtain a caustic solution of commercial value and yield a sodium chloride solution that can be recycled to the diaphragm cell.
A membrane cell, which employs a membrane selectively permeable to certain cations in place of a diaphragm, yields a catholyte of low salt content and having a caustic content of up to about 25 percent by weight or more. The highly corrosive caustic medium, however, is harsh on membrane materials and specifications for the membrane must be rigid. Membranes are quite complex and expensive. In addition, voltage drop within the membrane cell is relatively high which increases consumption of electricity. In sum, membrane cells are costly in regard to investment and operating costs.
Typical processes for concentrating cell liquor and separating the sodium chloride from the caustic involve evaporation and crystallization with the consumption of large amounts of steam and consequently, fuel required to generate steam. Investment in such processes is considerable.
One solution to the problem of obtaining cell liquor having high caustic concentration is described in U.S. Pat. No. 3,899,403 to Cook, Jr., et al. A three-compartment electrolytic cell produces solutions of high and low caustic concentration. A two-compartment cell then concentrates the solution of low caustic concentration. Current efficiency in the two-compartment cell is lower than that in the three-compartment cell. Overall current efficiency is about 70 percent. As both cells consume electric power, the energy cost of the process is high.
U.S. Pat. No. 4,036,717 to Babinsky et al describes a three-chamber electrolytic cell for concentrating and purifying cell liquor containing sodium or potassium hydroxide.
The Babinsky cell has a porous catalytic anode, a porous asbestos diaphragm between the anode chamber and a central chamber, and a cation-permselective membrane between the central chamber and the cathode chamber. Cell liquor is passed through the central chamber and concentrated caustic is withdrawn from the cathode chamber. Hydrogen gas generated by the electrolysis is supplied to the anode to decrease the potential across the cell below the evolution potential for chlorine and coincidentally reduce the power requirements for the cell.
The use of hydrogen-air fuel cells to consume hydrogen gas from chloralkali cells and to produce electricity for powering a portion of the chloralkali cells is described in Canadian Pat. No. 642,449. In the process described, hydrogen formed by the electrolysis of brine in a chloralkali cell is fed to the anode of a fuel cell and air is fed to the cathode. The fuel cell employs an aqueous electrolyte of sodium or potassium hydroxide having a concentration of 5 to 50 weight percent. There is no suggestion in the patent that the cell liquor from the chloralkali cells can be purified or concentrated by using such cell liquor as the electrolyte in the fuel cell.
U.S. Pat. No. 3,511,712 to Giner describes a process for removing carbon dioxide from gas streams using a fuel cell. An alkali metal carbonate solution formed by absorption of carbon dioxide is introduced to the anode compartment of a fuel cell operating by consumption of an oxidant and a fuel to generate hydrogen ions, consume hydroxyl ions and generate electricity. Operation decreases the pH of the electrolyte in the vicinity of the anode to a point where carbon dioxide is evolved with a simultaneous increase of pH. This restores the carbon dioxide absorptive capacity of the solution. The patent does not suggest the use of a fuel cell to purify and concentrate chloralkali cell liquor and, in fact, the Giner process would not be suitable for the treatment of cell liquor because the required lowering of the anolyte pH to 9 results in polarization of the anode and a severe lowering of the current efficiency of the cell.